Inter-small cell handover method, device, and system

ABSTRACT

The present application relates to the field of communications technologies, and provides an inter-small cell handover method, a device, and a system. When UE is in a coverage hole of a serving small cell, the UE needs to perform synchronous measurement only on each available beam pair in a set of available small cells determined by the UE, thereby reducing a delay of re-accessing a high frequency network by the UE, and improving QoS of receiving a high frequency service by the UE. the present application includes: performing, by UE, synchronous measurement, to determine a set of small cells available for the UE; sending the set of available small cells to a macro base station; when the UE is in a coverage hole of a current serving small cell, receiving, by the UE, a first synchronization indication sent by the macro base station.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/582,345, filed on Apr. 28, 2017, which is a continuation ofInternational Application No. PCT/CN2014/089943, filed on Oct. 30, 2014.All of the afore-mentioned patent applications are hereby incorporatedby reference in their entireties.

TECHNICAL FIELD

The present application relates to the field of communicationstechnologies, and in particular, to an inter-small cell handover method,a device, and a system.

BACKGROUND

With continuous development of broadband wireless communicationstechnologies, mobile broadband traffic in the future will be more than athousandfold of current mobile broadband traffic. To meet thedevelopment, using a high frequency band having a high rate and highbandwidth to perform data communication is bound to become a developmenttrend of a broadband wireless communications system. FIG. 1 is aschematic diagram of an architecture of a high frequency network. Asshown in FIG. 1, the high frequency network includes: one macro basestation, a user equipment (UE), and multiple small cells (including aserving small cell performing data communication with the UE) in acoverage area of the macro base station. A small cell may perform dataservice transmission with the UE by using a high frequency band.

Compared with a low frequency signal, a high frequency signal has aseverer transmission loss and a poorer penetration capability.Therefore, with impact of factors such as blocking of a building,blocking of a human body, and misalignment of a high frequency signalbeam, the UE is extremely easily located in a coverage hole of theserving small cell. Consequently, a high frequency signal received bythe UE is too poor to be demodulated, or the UE fails to receive a highfrequency signal sent by the serving small cell, thereby affectingquality of receiving a high frequency service by the UE and aprobability that the UE receives the high frequency service. Therefore,when the UE is in the coverage hole of the serving small cell, acorresponding measure needs to be taken, to reduce a probability thatreceiving a high frequency service by the UE is interrupted, andmaintain continuity of data communication.

To resolve the foregoing problem, currently, in a high frequency networkcommunications system, when the UE is in a coverage hole of a servingsmall cell, the UE performs synchronous measurement on all highfrequency beams of all small cells around the UE by using an initialsynchronous measurement method, selects an optimal small cell and beampair for re-access, and receives high frequency data sent by the smallcell, ensuring continuity of high frequency data communication. However,in this implementation method, the UE needs to perform synchronousmeasurement on all the beams of all the small cells, complexity isrelatively high, and a synchronous measurement time is relatively long,thereby leading to a relatively large delay of re-access of the UE toanother small cell, and reducing quality of service (QoS) of the UE.

SUMMARY

Embodiments of the present application provide an inter-small cellhandover method, a device, and a system, so as to resolve a problem thatwhen the UE is in a coverage hole of a serving small cell, the UEperforms synchronous measurement on all beams of all small cells,causing a relatively large access delay, and relatively low QoS ofreceiving a high frequency service by the UE.

To achieve the foregoing objective, the following technical solutionsare used in the embodiments of the present application:

According to a first aspect, an embodiment of the present applicationprovides an inter-small cell handover method, including:

performing, by the UE, synchronous measurement, to determine a set ofsmall cells available for the UE, where the set of available small cellsincludes at least one available small cell, and an available beam pairof each available small cell, and the available beam pair consists of ahigh frequency beam of the UE and a high frequency beam of the availablesmall cell;

sending, by the UE, the set of available small cells to a macro basestation;

when the UE is in a coverage hole of a current serving small cell,receiving a first synchronization indication sent by the macro basestation, where the first synchronization indication is used to instructthe UE to perform synchronous measurement on the high frequency beam ofthe UE in each available beam pair;

performing, by the UE, synchronous measurement according to the firstsynchronization indication, to select an available target beam pair; and

handing over, by the UE, to a target small cell corresponding to theavailable target beam pair.

According to a second aspect, an embodiment of the present applicationprovides an inter-small cell handover method, including:

receiving, by a macro base station, a set of small cells available forUE that is sent by the UE, where the set of available small cellsincludes at least one available small cell, and an available beam pairof each available small cell, and the available beam pair consists of ahigh frequency beam of the UE and a high frequency beam of the availablesmall cell;

receiving, by the macro base station, a coverage hole status indicationsent by a current serving small cell of the UE, where the coverage holestatus indication is used to indicate whether the UE is in a coveragehole of the current serving small cell; and

when the UE is in the coverage hole of the current serving small cell,sending, by the macro base station, a first synchronization indicationto the UE, and separately sending a second synchronization indication toeach available small cell in the set of available small cells, so thatthe UE performs synchronous measurement according to the firstsynchronization indication, selects an available target beam pair, andhands over from the current serving small cell of the UE to a targetsmall cell corresponding to the available target beam pair, where thefirst synchronization indication is used to instruct the UE to performsynchronous measurement on the high frequency beam of the UE in eachavailable beam pair, and the second synchronization indication is usedto instruct the available small cell to send a synchronization pilotsignal to the UE on the high frequency beam in the available beam pairof the available small cell.

According to a third aspect, an embodiment of the present applicationprovides an inter-small cell handover method, including:

sending a first synchronization pilot signal to the UE, so that the UEmeasures the synchronization pilot signal, and determines an availablesmall cell of the UE and an available beam pair of the available smallcell, where the available beam pair consists of a high frequency beam ofthe available small cell and a high frequency beam of the UE;

receiving, by the available small cell, a synchronization indicationsent by a macro base station, where the synchronization indicationinstructs the available small cell to send a synchronization pilotsignal to the UE on the high frequency beam in the available beam pairof the available small cell;

sending, by the available small cell, a second synchronization pilotsignal to the UE according to the synchronization indication, so thatthe UE performs synchronous measurement on the high frequency beam ofthe UE in the available beam pair, determines an available target beampair, and hands over from a current serving small cell of the UE to atarget small cell corresponding to the available target beam pair; and

receiving, by the target small cell, a data transmission indication sentby the macro base station, to perform data transmission with the UE byusing the available target beam pair.

According to a fourth aspect, an embodiment of the present applicationprovides user equipment, where the user equipment includes:

a determining unit, configured to perform synchronous measurement, todetermine a set of small cells available for the UE, where the set ofavailable small cells includes at least one available small cell, and anavailable beam pair of each available small cell, and the available beampair consists of a high frequency beam of the UE and a high frequencybeam of the available small cell;

a sending unit, configured to send the set of available small cells to amacro base station;

a receiving unit, configured to: when the UE is in a coverage hole of acurrent serving small cell, receive a first synchronization indicationsent by the macro base station, where the first synchronizationindication is used to instruct the UE to perform synchronous measurementon the high frequency beam of the UE in each available beam pair;

a selection unit, configured to perform synchronous measurementaccording to the first synchronization indication, to select anavailable target beam pair; and

a handover unit, configured to hand over to a target small cellcorresponding to the available target beam pair.

According to a fifth aspect, an embodiment of the present applicationprovides a macro base station, where the macro base station includes:

a receiving unit, configured to: receive a set of available small cellssent by the UE, where the set of available small cells includes at leastone available small cell, and an available beam pair of each availablesmall cell, and the available beam pair consists of a high frequencybeam of the UE and a high frequency beam of the available small cell;and

receive a coverage hole status indication sent by a serving small cellof the UE, where the coverage hole status indication is used to indicatewhether the UE is in a coverage hole of the serving small cell; and

a sending unit, configured to: when the UE is in the coverage hole ofthe current serving small cell, send a first synchronization indicationto the UE, and separately send a second synchronization indication toeach available small cell in the set of available small cells, so thatthe UE performs synchronous measurement according to the firstsynchronization indication, selects an available target beam pair, andhands over from the current serving small cell of the UE to a targetsmall cell corresponding to the available target beam pair, where thefirst synchronization indication is used to instruct the UE to performsynchronous measurement on the high frequency beam of the UE in eachavailable beam pair, and the second synchronization indication is usedto instruct the available small cell to send a synchronization pilotsignal to the UE on the high frequency beam in the available beam pairof the available small cell.

According to a sixth aspect, an embodiment of the present applicationprovides a small cell, where the small cell includes:

a sending unit, configured to send a first synchronization pilot signalto the UE, so that the UE measures the synchronization pilot signal, anddetermines an available small cell of the UE and an available beam pairof the available small cell, where the available beam pair consists of ahigh frequency beam of the available small cell and a high frequencybeam of the UE;

a receiving unit, configured to receive a synchronization indicationsent by a macro base station, where the synchronization indicationinstructs the available small cell to send a synchronization pilotsignal to the UE on the high frequency beam in the available beam pairof the available small cell, where

the sending unit is further configured to send a second synchronizationpilot signal to the UE according to the synchronization indication, sothat the UE performs synchronous measurement on the high frequency beamof the UE in the available beam pair, determines an available targetbeam pair, and hands over from a current serving small cell of the UE toa target small cell corresponding to the available target beam pair; and

the receiving unit is further configured to receive a data transmissionindication sent by the macro base station; and

a data communications unit, configured to perform data transmission withthe UE by using the available target beam pair.

According to a seventh aspect, an embodiment of the present applicationprovides an inter-small cell handover system, including the userequipment according to any one of the fourth aspect or the first to thethird possible implementation manners of the fourth aspect, the macrobase station according to any one of the fifth aspect or the first orthe second possible implementation manner of the fifth aspect, and atleast one small cell according to any one of the sixth aspect or thefirst to the third possible implementation manners of the sixth aspect.

According to an eighth aspect, an embodiment of the present applicationprovides user equipment, where the user equipment includes:

a processor, configured to perform synchronous measurement, to determinea set of small cells available for the UE, where the set of availablesmall cells includes at least one available small cell, and an availablebeam pair of each available small cell, and the available beam pairconsists of a high frequency beam of the UE and a high frequency beam ofthe available small cell;

a communications unit, configured to: send the set of available smallcells to a macro base station; and

when the UE is in a coverage hole of a current serving small cell,receive a first synchronization indication sent by the macro basestation, where the first synchronization indication is used to instructthe UE to perform synchronous measurement on the high frequency beam ofthe UE in each available beam pair; and

the processor is further configured to perform synchronous measurementaccording to the first synchronization indication, to select anavailable target beam pair; and

hand over to a target small cell corresponding to the available targetbeam pair.

According to a ninth aspect, an embodiment of the present applicationprovides a macro base station, where the macro base station includes:

a communications unit, configured to: receive a set of available smallcells sent by the UE, where the set of available small cells includes atleast one available small cell, and an available beam pair of eachavailable small cell, and the available beam pair consists of a highfrequency beam of the UE and a high frequency beam of the availablesmall cell; and

receive a coverage hole status indication sent by a serving small cellof the UE, where the coverage hole status indication is used to indicatewhether the UE is in a coverage hole of the serving small cell; and

when the UE is in the coverage hole of the current serving small cell,send a first synchronization indication to the UE, and separately send asecond synchronization indication to each available small cell in theset of available small cells, so that the UE performs synchronousmeasurement according to the first synchronization indication, selectsan available target beam pair, and hands over from the current servingsmall cell of the UE to a target small cell corresponding to theavailable target beam pair, where the first synchronization indicationis used to instruct the UE to perform synchronous measurement on thehigh frequency beam of the UE in each available beam pair, and thesecond synchronization indication is used to instruct the availablesmall cell to send a synchronization pilot signal to the UE on the highfrequency beam in the available beam pair of the available small cell.

According to a tenth aspect, an embodiment of the present applicationprovides a small cell, where the small cell includes:

a communications unit, configured to: send a first synchronization pilotsignal to the UE, so that the UE measures the synchronization pilotsignal, and determines an available small cell of the UE and anavailable beam pair of the available small cell, where the availablebeam pair consists of a high frequency beam of the available small celland a high frequency beam of the UE;

receive a synchronization indication sent by a macro base station, wherethe synchronization indication instructs the available small cell tosend a synchronization pilot signal to the UE on the high frequency beamin the available beam pair of the available small cell;

send a second synchronization pilot signal to the UE according to thesynchronization indication, so that the UE performs synchronousmeasurement on the high frequency beam of the UE in the available beampair, determines an available target beam pair, and hands over from acurrent serving small cell of the UE to a target small cellcorresponding to the available target beam pair;

receive a data transmission indication sent by the macro base station;and

perform data transmission with the UE by using the available target beampair.

According to an eleventh aspect, an embodiment of the presentapplication provides an inter-small cell handover system, including theuser equipment according to any one of the eighth aspect or the first tothe third possible implementation manners of the eighth aspect, themacro base station according to any one of the ninth aspect or the firstor the second possible implementation manner of the ninth aspect, andthe small cell according to any one of the tenth aspect or the first tothe third possible implementation manners of the tenth aspect.

It can be known from the foregoing that, by means of the inter-smallcell handover method and the device that are provided in the embodimentsof the present application, the UE performs synchronous measurement, todetermine a set of small cells available for the UE; sends the set ofavailable small cells to a macro base station; when the UE is in acoverage hole of a serving small cell, receives a first synchronizationindication sent by the macro base station; performs synchronousmeasurement according to the first synchronization indication, to selectan available target beam pair; and hands over to a target small cellcorresponding to the available target beam pair. In this way, when theUE is in the coverage hole of the serving small cell, as long as the UEperforms synchronous measurement on each available beam pair in the setof available small cells determined by the UE, the UE can select anoptimal target small cell and target beam pair, and hands over to thetarget small cell, thereby reducing a delay of re-accessing a highfrequency network by the UE, improving QoS of receiving a high frequencyservice by the UE, and avoiding a problem in the prior art that the UEneeds to perform synchronous measurement on all beam pairs of all smallcells, and a synchronous measurement time is long, causing a relativelylarge delay of re-accessing a high frequency network by the UE, andrelatively poor QoS of receiving a high frequency service by the UE.

BRIEF DESCRIPTION OF DRAWINGS

To describe the technical solutions in the embodiments of the presentapplication or in the prior art more clearly, the following brieflydescribes the accompanying drawings required for describing theembodiments or the prior art. Apparently, the accompanying drawings inthe following description show merely some embodiments of the presentapplication, and persons of ordinary skill in the art may still deriveother drawings from these accompanying drawings without creativeefforts.

FIG. 1 is a schematic diagram of an architecture of a high frequencynetwork;

FIG. 2 is a flowchart of an inter-small cell handover method accordingto an embodiment of the present application;

FIG. 3 is a flowchart of an inter-small cell handover method accordingto an embodiment of the present application;

FIG. 4 is a flowchart of an inter-small cell handover method accordingto an embodiment of the present application;

FIG. 5 is a flowchart of an inter-small cell handover method accordingto an embodiment of the present application;

FIG. 6 is a structural diagram of user equipment 60 according to anembodiment of the present application;

FIG. 6A is a structural diagram of user equipment 60 according to anembodiment of the present application;

FIG. 7 is a structural diagram of a macro base station 70 according toan embodiment of the present application;

FIG. 7A is a structural diagram of a macro base station 70 according toan embodiment of the present application;

FIG. 7B is a structural diagram of a macro base station 70 according toan embodiment of the present application;

FIG. 8 is a structural diagram of a small cell 80 according to anembodiment of the present application;

FIG. 8A is a structural diagram of a small cell 80 according to anembodiment of the present application;

FIG. 9 is a structural diagram of an inter-small cell handover system 90according to an embodiment of the present application;

FIG. 10 is a structural diagram of user equipment 100 according to anembodiment of the present application;

FIG. 11 is a structural diagram of a macro base station 110 according toan embodiment of the present application;

FIG. 12 is a structural diagram of a small cell 120 according to anembodiment of the present application; and

FIG. 13 is a structural diagram of an inter-small cell handover system130 according to an embodiment of the present application.

DESCRIPTION OF EMBODIMENTS

The following clearly describes the technical solutions in theembodiments of the present application with reference to theaccompanying drawings in the embodiments of the present application.Apparently, the described embodiments are merely some but not all of theembodiments of the present application. All other embodiments obtainedby persons of ordinary skill in the art based on the embodiments of thepresent application without creative efforts shall fall within theprotection scope of the present application.

An inter-small cell handover method provided in an embodiment of thepresent application is applicable to a high frequency networkarchitecture shown in FIG. 1, and is also applicable to an inter-smallcell handover in any other coordinated communications scenario. This isnot limited in the present application. Only an example of aninter-small cell handover in the high frequency network architectureshown in FIG. 1 is used in this embodiment of the present applicationfor description.

In the high frequency network architecture shown in FIG. 1, a small cell1 is a serving small cell of the UE, and a small cell 2, a small cell 3,and a small cell 4 are small cells that are in a coverage area of amacro base station and that are neighboring to the serving small cell ofthe UE. Each small cell can separately generate 10 high frequency beams,and the UE can generate four high frequency beams: a beam 1, a beam 2, abeam 3, and a beam 4. When the UE is in a coverage hole of the servingsmall cell 1, if an existing method for reducing an interruptionprobability of high frequency data communication of the UE is used, theUE needs to separately measure, on the beam 1 of the UE, synchronizationpilot signals that are sent by the small cell 2 on 10 high frequencybeams of the small cell 2, by the small cell 3 on 10 high frequencybeams of the small cell 3, and by the small cell 4 on 10 high frequencybeams of the small cell 4; the UE needs to separately measure, on thebeam 2 of the UE, the synchronization pilot signals that are sent by thesmall cell 2 on the 10 high frequency beams of the small cell 2, by thesmall cell 3 on the 10 high frequency beams of the small cell 3, and bythe small cell 4 on the 10 high frequency beams of the small cell 4; theUE needs to separately measure, on the beam 3 of the UE, thesynchronization pilot signals that are sent by the small cell 2 on the10 high frequency beams of the small cell 2, by the small cell 3 on the10 high frequency beams of the small cell 3, and by the small cell 4 onthe 10 high frequency beams of the small cell 4; and the UE needs toseparately measure, on the beam 4 of the UE, the synchronization pilotsignals that are sent by the small cell 2 on the 10 high frequency beamsof the small cell 2, by the small cell 3 on the 10 high frequency beamsof the small cell 3, and by the small cell 4 on the 10 high frequencybeams of the small cell 4, select a small cell and a beam pair thatcorrespond to a synchronization pilot signal having a highest receivedsignal strength, access the selected small cell, and perform highfrequency data transmission by using the selected beam pair.

It is not difficult to understand that, in the high frequency networkarchitecture shown in FIG. 1, when the UE is in a coverage hole of aserving small cell, if an existing inter-small cell handover technologyis used, the UE cannot select an optimal small cell and beam pair tore-access a high frequency network, until the UE performs 4×3×10=120times of synchronous measurement, that is, a quantity of times ofsynchronous measurement performed by the UE in an inter-small cellhandover process has a great relationship with a quantity of small cellsaround the UE and a quantity of high frequency beams that can begenerated by each small cell. In this case, the quantity of small cellsaround the UE or the quantity of high frequency beams generated by eachsmall cell or both are extremely large. If the UE needs to performsynchronous measurement with all neighboring small cells of the servingsmall cell to select an optimal available small cell, a quantity oftimes of synchronous measurement is relatively large, and a synchronousmeasurement time is relatively long, thereby causing a relatively largedelay of re-accessing another small cell by the UE, and reducing QoS ofreceiving a high frequency service by the UE. Therefore, this embodimentof the present application provides the inter-small cell handovermethod. The inter-small cell handover method is applied to the highfrequency network architecture shown in FIG. 1. Referring to FIG. 2, themethod may include the following steps.

201. The UE performs synchronous measurement, to determine a set ofsmall cells available for the UE.

The set of available small cells includes at least one available smallcell, and an available beam pair of each available small cell, and theavailable beam pair consists of a high frequency beam of the UE and ahigh frequency beam of the available small cell.

Preferably, the UE may perform synchronous measurement upon beingpowered on, to determine the set of small cells available for the UEwhile a serving small cell of the UE is determined; or may performsynchronous measurement when a user requests to establish a connection,to determine a serving small cell of the UE and the set of small cellsavailable for the UE; or may periodically perform synchronousmeasurement while the UE receives high frequency data sent by a servingsmall cell, to determine the set of small cells available for the UE.

Specifically, the UE may use the following method to perform synchronousmeasurement, to determine the serving small cell of the UE and the setof available small cells:

sending, by the UE, a synchronous measurement request to the macro basestation, so that the macro base station instructs all small cells in acoverage area of the macro base station to send synchronization pilotsignals to the UE;

separately receiving, by the UE on each high frequency beam of the UE,the synchronization pilot signal sent by all the small cells on eachhigh frequency beam of the small cells;

separately measuring, by the UE, a signal strength of eachsynchronization pilot signal, and sorting the signal strengths indescending order; and

determining, by the UE, a small cell sending a synchronization pilotsignal having a highest signal strength as a serving small cell of theUE; and determining at least one small cell sending a synchronizationpilot signal having a second highest signal strength as an availablesmall cell in the set of small cells available for the UE.

The signal strength may be a measured RSRP (reference signal receivedpower) value of a reference signal of a current neighboring small cellor a measured RSRQ (Reference Signal Received Quality) value of areference signal of a current neighboring small cell.

For example, as shown in FIG. 1, after performing initial synchronousmeasurement, the UE determines the small cell 1 as the serving smallcell of the UE, determines that available small cells in the set ofavailable small cells are the small cell 2 and the small cell 3,determines that an available beam pair corresponding to the small cell 2is (a high frequency beam 2 of the UE, a high frequency beam 4 of thesmall cell 2), and determines that an available beam pair correspondingto the small cell 3 is (a high frequency beam 3 of the UE, a highfrequency beam 6 of the small cell 3).

202. The UE sends the set of available small cells to the macro basestation.

Preferably, the UE may send the set of available small cells to themacro base station by using a low frequency channel, so that the macrobase station identifies the UE according to information about whetherthe received set of available small cells includes an available smallcell. Preferably, if an available small cell is included, the UE isidentified as a user that can hand over between small cells, so as toinitiate the inter-small cell handover process in the presentapplication when the UE is in the coverage hole of the serving smallcell. If no available small cell is included, the UE is considered bydefault as or is identified as a user that cannot hand over betweensmall cells.

203. When the UE is in a coverage hole of a current serving small cell,receive a first synchronization indication sent by the macro basestation.

The first synchronization indication is used to instruct the UE toperform synchronous measurement on the high frequency beam of the UE ineach available beam pair.

Preferably, the UE may receive, on a low frequency channel, the firstsynchronization indication sent by the macro base station.

For example, if the available beam pair determined in step 201 is (thehigh frequency beam 2 of the UE, the high frequency beam 4 of the smallcell 2) and (the high frequency beam 3 of the UE, the high frequencybeam 6 of the small cell 3), the first synchronization indication isused to instruct the UE to separately receive, on the high frequencybeam 2 and the high frequency beam 3 of the UE, a synchronization pilotsignal sent by a small cell, to perform synchronous measurement.

204. The UE performs synchronous measurement according to the firstsynchronization indication, to select an available target beam pair.

The available target beam pair is a beam pair having a good channelquality and meeting a high frequency data transmission requirement.

Preferably, the UE may use the following method to select the availabletarget beam pair:

measuring, by the UE, a signal strength of a synchronization pilotsignal on the high frequency beam of the UE in each available beam pair;and

if the signal strength corresponding to the synchronization pilot signalhaving the highest signal strength is greater than or equal to a presetthreshold, selecting, as the available target beam pair, a beam pairconsisting of a high frequency beam, of the UE, on which thesynchronization pilot signal having the highest signal strength isreceived and a high frequency beam of an available small cell sendingthe synchronization pilot signal having the highest signal strength.

If the signal strength of the synchronization pilot signal having thehighest signal strength is less than or equal to the preset threshold,the UE uses an existing inter-small cell handover method to performsynchronous measurement on all beams of all small cells, to select anoptimal beam pair.

The preset threshold is set as required, and is not limited in thisembodiment of the present application. If a signal strength of asynchronization pilot signal is greater than or equal to the presetthreshold, it indicates that quality of a channel on which thesynchronization pilot signal is sent and received meets a requirement ofa high frequency data transmission channel in the high frequency networkarchitecture. If a signal strength of a synchronization pilot signal isless than the preset threshold, it indicates that quality of a channelon which the synchronization pilot signal is sent and received cannotmeet a requirement of a high frequency data transmission channel in thehigh frequency network architecture, and the channel is an unavailablehigh frequency transmission channel.

For example, when the UE in FIG. 1 is in the coverage hole of theserving small cell, the UE needs to perform synchronous measurement onlyon the available beam pairs (the high frequency beam 2 of the UE, thehigh frequency beam 4 of the small cell 2) and (the high frequency beam3 of the UE, the high frequency beam 6 of the small cell 3), to selectan available target beam pair from the two beam pairs, that is, anoptimal beam pair can be determined as long as synchronous measurementis performed twice. Compared with an existing method in which an optimalbeam pair cannot be determined until synchronous measurement isperformed 120 times, this greatly reduces a quantity of times ofsynchronous measurement, and reduces a synchronous measurement time.Therefore, a time is saved for a small cell handover of the UE in step205, a delay of re-accessing a high frequency network by the UE isreduced, and QoS of receiving a high frequency service by the UE isimproved.

205. The UE hands over to a target small cell corresponding to theavailable target beam pair.

Further, the method further includes:

sending, by the UE, a handover result to the macro base station, so thatthe macro base station instructs, according to the handover result, toperform data transmission between the target small cell and the UE byusing the available target beam pair, and instructs the serving smallcell to terminate data transmission with the UE, where the handoverresult includes the available target beam pair and a cell identifier ofthe target small cell; and

receiving a data transmission indication sent by the macro base station,to perform data transmission with the target small cell by using theavailable target beam pair, where the data transmission indication isused to instruct to perform data transmission between the target smallcell and the UE by using the available target beam pair.

It can be known from the foregoing that, by means of the inter-smallcell handover method provided in this embodiment of the presentapplication, the UE performs synchronous measurement, to determine a setof small cells available for the UE; sends the set of available smallcells to a macro base station; when the UE is in a coverage hole of acurrent serving small cell, performs synchronous measurement accordingto a first synchronization indication, to select an available targetbeam pair; and hands over to a target small cell corresponding to theavailable target beam pair. In this way, when the UE is in the coveragehole of the serving small cell, as long as the UE performs synchronousmeasurement on each available beam pair in the set of available smallcells determined by the UE, the UE can select an optimal small cell andbeam pair, access the selected small cell, and continuously perform highfrequency data transmission by using the selected beam pair, therebyreducing a synchronous measurement time, further reducing a delay ofre-accessing a high frequency network by the UE, improving QoS ofreceiving a high frequency service by the UE, and avoiding a problem inthe prior art that the UE needs to perform synchronous measurement onall beam pairs of all small cells, and a synchronous measurement time islong, causing a relatively large delay of re-accessing a high frequencynetwork by the UE, and relatively poor QoS of receiving a high frequencyservice by the UE.

In addition, an embodiment of the present application further providesan inter-small cell handover method. As shown in FIG. 3, the method mayinclude the following steps.

301. A macro base station receives a set of available small cells sentby the UE.

The set of available small cells includes at least one available smallcell, and an available beam pair of each available small cell, and theavailable beam pair consists of a high frequency beam of the UE and ahigh frequency beam of the available small cell.

302. The macro base station receives a coverage hole status indicationsent by a serving small cell of the UE.

The coverage hole status indication is used to indicate whether the UEis in a coverage hole of the serving small cell.

Preferably, the macro base station may receive, on a low frequencychannel, the coverage hole status indication sent by the serving smallcell.

303. When the UE is in a coverage hole of a current serving small cell,the macro base station sends a first synchronization indication to theUE, and separately sends a second synchronization indication to eachavailable small cell in the set of available small cells, so that the UEperforms synchronous measurement according to the first synchronizationindication, to select an available target beam pair.

The first synchronization indication is used to instruct the UE toperform synchronous measurement on the high frequency beam of the UE ineach available beam pair. The second synchronization indication is usedto instruct the available small cell to send a synchronization pilotsignal to the UE on the high frequency beam in the available beam pairof the available small cell.

Further, the method further includes:

receiving, by the macro base station, a handover result sent by the UE,where the handover result includes the available target beam pair and acell identifier of a target small cell corresponding to the availabletarget beam pair; and

instructing, according to the handover result, to perform datatransmission between the target small cell and the UE by using theavailable target beam pair, and instructing the serving small cell toterminate data transmission with the UE.

Further, to ensure that the UE receives, as quickly as possible when theUE is in the coverage hole of the current serving small cell, datatransmitted by the target small cell on a high frequency channel, andreduces a connection delay of a high frequency data service, before themacro base station instructs the target small cell to perform datatransmission to the UE by using the available target beam pair, themethod may further include:

grouping, by the macro base station into one group, an available smallcell that is in the set of available small cells and that is in a samecoverage area of the macro base station; and

after receiving the coverage hole status indication sent by the servingsmall cell of the UE, separately sending, by the macro base station in amulticast form to each group of available small cells, data that the UErequests to transmit.

In this way, before synchronous measurement is performed on a UE side todetermine the available target beam pair, the macro base station firstsends, in a multicast form to each small cell in the set of availablesmall cells by means of data transmission, the data requested by the UE,so that after receiving the handover result, the macro base stationdirectly commands the target small cell to perform data transmission tothe UE, rather than transmitting, to the target small cell after thehandover result is received, the data requested by the UE, to enable thetarget small cell to perform data transmission to the UE.

It can be known from the foregoing that, by means of the inter-smallcell handover method provided in this embodiment of the presentapplication, after a macro base station receives a set of availablesmall cells sent by the UE and a coverage hole status indication sent bya current serving small cell of the UE, when the UE is in a coveragehole of the current serving small cell, the macro base station sends afirst synchronization indication to the UE, and separately sends asecond synchronization indication to each available small cell in theset of available small cells, so that the UE performs synchronousmeasurement according to the first synchronization indication, to selectan available target beam pair. The first synchronization indication isused to instruct the UE to perform synchronous measurement on a highfrequency beam of the UE in each available beam pair, and the secondsynchronization indication is used to instruct the available small cellto send a synchronization pilot signal to the UE on a high frequencybeam in the available beam pair of the available small cell. In thisway, after the macro base station receives information that is sent bythe serving small cell and that the UE is in a coverage hole of theserving small cell, the macro base station commands the UE to performsynchronous measurement on the available beam pair in the determined setof available small cells, so that a quantity of times of synchronousmeasurement performed by the UE is greatly reduced, and a measurementtime is reduced. Therefore, a delay of re-accessing a high frequencynetwork by the UE is reduced, and high frequency QoS of the UE isimproved.

In addition, an embodiment of the present application further providesan inter-small cell handover method. As shown in FIG. 4, the method mayinclude the following steps.

401. Send a first synchronization pilot signal to the UE, so that the UEmeasures the synchronization pilot signal, and determines an availablesmall cell of the UE and an available beam pair of the available smallcell.

The available beam pair consists of a high frequency beam of theavailable small cell and a high frequency beam of the UE.

402. The available small cell receives a synchronization indication sentby a macro base station, where the synchronization indication instructsthe available small cell to send a synchronization pilot signal to theUE on a high frequency beam in the available beam pair of the availablesmall cell.

403. The available small cell sends a second synchronization pilotsignal to the UE according to the synchronization indication, so thatthe UE performs synchronous measurement on a high frequency beam of theUE in the available beam pair, determines an available target beam pair,and hands over from a current serving small cell of the UE to a targetsmall cell corresponding to the available target beam pair.

404. The target small cell receives a data transmission indication sentby the macro base station, to perform data transmission with the UE byusing the available target beam pair.

Further, before step 404, the method further includes:

receiving, by the available small cell, data that is sent by the macrobase station in a multicast form and that the UE requests to transmit.

Further, when the small cell is the current serving small cell of theUE, the method further includes:

determining whether the UE is in a coverage hole of the current servingsmall cell;

sending a coverage hole status indication to the macro base station,where the coverage hole status indication is used to indicate whetherthe UE is in the coverage hole of the small cell; and

when the UE is in the coverage hole of the current serving small cell,receiving, by the current serving small cell, indication informationsent by the macro base station, to terminate data transmission with theUE.

Preferably, whether the UE is in the coverage hole of the serving smallcell may be determined by using any one of the following three methods(1), (2), or (3):

(1) if it is continuously determined at least twice that an uplink ofthe UE is incorrectly demodulated, determining that the UE is in thecoverage hole of the current serving small cell;

(2) if a received reference signal received power RSRP or referencesignal received quality RSRQ sent by the UE is less than a presetthreshold, determining that the UE is in the coverage hole of thecurrent serving small cell, where

the preset threshold is set as required, and is not limited in thisembodiment of the present application; or

(3) if a response message of the UE is not received continuously atleast twice after a request message is sent to the UE, determining thatthe UE is in the coverage hole of the small cell.

It can be known from the foregoing that, by means of the inter-smallcell handover method provided in this embodiment of the presentapplication, a first synchronization pilot signal is sent to the UE, sothat the UE measures the synchronization pilot signal, and determines anavailable small cell of the UE and an available beam pair of theavailable small cell, where the available beam pair consists of a highfrequency beam of the available small cell and a high frequency beam ofthe UE; the available small cell receives a synchronization indicationsent by a macro base station, where the synchronization indicationinstructs the available small cell to send a synchronization pilotsignal to the UE on the high frequency beam in the available beam pairof the available small cell; the available small cell sends a secondsynchronization pilot signal to the UE according to the synchronizationindication, so that the UE performs synchronous measurement on the highfrequency beam of the UE in the available beam pair, determines anavailable target beam pair, and hands over from a current serving smallcell of the UE to a target small cell corresponding to the availabletarget beam pair; the target small cell receives a data transmissionindication sent by the macro base station, to perform data transmissionwith the UE by using the available target beam pair. In this way, whenthe UE is in a coverage hole of the serving small cell, the UE needs toperform synchronous measurement only on the available beam pair in a setof available small cells, greatly reducing a quantity of synchronizationtimes. Therefore, a delay of accessing a high frequency network by theUE is reduced, and QoS of receiving a high frequency service by the UEis improved.

An example of an inter-small cell handover in the high frequency networkarchitecture shown in FIG. 1 is used below to describe the foregoingmethod in detail. It is assumed that a current serving small cell of theUE is a small cell 1, a set of available small cells includes a smallcell 2 and a small cell 3, and when the UE is in a coverage hole of theserving small cell, a determined target small cell is the small cell 2.

FIG. 5 is a flowchart of an inter-small cell handover method accordingto an embodiment of the present application. As shown in FIG. 5, themethod may include the following steps.

501. The UE performs synchronous measurement, to determine a set ofsmall cells available for the UE.

The set of available small cells includes a small cell 2, a small cell3, an available beam pair (a high frequency beam 2 of the UE, a highfrequency beam 4 of the small cell 2) of the small cell 2, and anavailable beam pair (a high frequency beam 3 of the UE, a high frequencybeam 6 of the small cell 3) of the small cell 3.

502. The UE sends the set of available small cells to a macro basestation.

503. A small cell 1 determines that the UE is in a coverage hole of aserving small cell.

Preferably, the small cell 1 may determine, by using any one of thefollowing three methods (1), (2), or (3), that the UE is in the coveragehole of the serving small cell:

(1) if it is continuously determined at least twice that an uplink ofthe UE is incorrectly demodulated, determining that the UE is in thecoverage hole of the current serving small cell;

(2) if a received reference signal received power RSRP or referencesignal received quality RSRQ sent by the UE is less than a presetthreshold, determining that the UE is in the coverage hole of thecurrent serving small cell, where

the preset threshold is set as required, and is not limited in thisembodiment of the present application; or

(3) if a response message of the UE is not received continuously atleast twice after a request message is sent to the UE, determining thatthe UE is in the coverage hole of the small cell.

504. The small cell 1 sends a coverage hole status indication to themacro base station.

505. The macro base station sends a first synchronization indication tothe UE, and separately sends a second synchronization indication to eachavailable small cell in the set of available small cells.

The first synchronization indication is used to instruct the UE toseparately perform synchronous measurement on the high frequency beam ofthe UE in each available beam pair. The second synchronizationindication is used to instruct the available small cell to send asynchronization pilot signal to the UE on the high frequency beam in theavailable beam pair of the available small cell.

506. The UE performs synchronous measurement according to the firstsynchronization indication, to select an available target beam pair (ahigh frequency beam 2 of the UE, a high frequency beam 4 of a small cell2).

507. The UE hands over to the small cell 2: a target small cellcorresponding to the available target beam pair.

508. The UE sends a handover result to the macro base station.

509. The macro base station sends indication information to the smallcell 2 according to the handover result, where the indicationinformation is used to instruct the small cell 2 to transmit highfrequency data with the UE by using the available target beam pair.

It can be known from the foregoing that, by means of the inter-smallcell handover method provided in this embodiment of the presentapplication, the UE performs synchronous measurement, determines a setof small cells available for the UE, and sends the set of availablesmall cells to a macro base station; when determining that the UE is ina coverage hole of a serving small cell, the serving small cell sends acoverage hole status indication to the macro base station; the macrobase station sends a first synchronization indication to the UE, andseparately sends a second synchronization indication to an availablesmall cell in the set of available small cells; the UE performssynchronous measurement, selects a target beam pair, hands over to atarget small cell corresponding to the target beam pair, and sends ahandover result to the macro base station; the macro base stationinstructs the target small cell to transmit high frequency data to theUE. In this way, when the UE is in the coverage hole of the servingsmall cell, as long as the UE performs synchronous measurement on eachavailable beam pair in the set of available small cells determined bythe UE, the UE can select an optimal target small cell and target beampair, and hands over to the target small cell, thereby reducing a delayof re-accessing a high frequency network by the UE, improving QoS ofreceiving a high frequency service by the UE, and avoiding a problem inthe prior art that the UE needs to perform synchronous measurement onall beam pairs of all small cells, and a synchronous measurement time islong, causing a relatively large delay of re-accessing a high frequencynetwork by the UE, and relatively poor QoS of receiving a high frequencyservice by the UE.

In addition, an embodiment of the present application further providesuser equipment 60. As shown in FIG. 6, the user equipment 60 mayinclude:

a determining unit 601, configured to perform synchronous measurement,to determine a set of small cells available for the UE, where

the set of available small cells includes at least one available smallcell, and an available beam pair of each available small cell, and theavailable beam pair consists of a high frequency beam of the UE and ahigh frequency beam of the available small cell;

a sending unit 602, configured to send the set of available small cellsto a macro base station;

a receiving unit 603, configured to: when the UE is in a coverage holeof a current serving small cell, receive a first synchronizationindication sent by the macro base station, where

the first synchronization indication is used to instruct the UE toperform synchronous measurement on the high frequency beam of the UE ineach available beam pair;

a selection unit 604, configured to perform synchronous measurementaccording to the first synchronization indication, to select anavailable target beam pair; and

a handover unit 605, configured to hand over to a target small cellcorresponding to the available target beam pair.

Further, the determining unit 601 is specifically configured to:

send a synchronous measurement request to the macro base station, sothat the macro base station instructs all small cells in a coverage areaof the macro base station to send synchronization pilot signals to theUE;

separately receive, on each high frequency beam of the UE, thesynchronization pilot signal sent by all the small cells on each highfrequency beam of the small cells;

separately measure a signal strength of each synchronization pilotsignal, and sort the signal strengths in descending order; and

determine a small cell sending a synchronization pilot signal having ahighest signal strength as a serving small cell of the UE; and determineat least one small cell sending a synchronization pilot signal having asecond highest signal strength as an available small cell in the set ofsmall cells available for the UE.

The signal strength may be a measured RSRP value of a reference signalof a current neighboring small cell or a measured RSRQ value of areference signal of a current neighboring small cell.

For example, as shown in FIG. 1, after performing synchronousmeasurement, the UE determines the small cell 1 as the serving smallcell of the UE, determines that available small cells in the set ofavailable small cells are the small cell 2 and the small cell 3,determines that an available beam pair corresponding to the small cell 2is (a high frequency beam 2 of the UE, a high frequency beam 4 of thesmall cell 2), and determines that an available beam pair correspondingto the small cell 3 is (a high frequency beam 3 of the UE, a highfrequency beam 6 of the small cell 3).

Further, the selection unit 604 is specifically configured to:

measure a signal strength of a synchronization pilot signal on the highfrequency beam of the UE in each available beam pair; and

if the signal strength corresponding to the synchronization pilot signalhaving the highest signal strength is greater than or equal to a presetthreshold, select, as the available target beam pair, a beam pairconsisting of a high frequency beam, of the UE, on which thesynchronization pilot signal having the highest signal strength isreceived and a high frequency beam of an available small cell sendingthe synchronization pilot signal having the highest signal strength.

The preset threshold is set as required, and is not limited in thisembodiment of the present application. If a signal strength of asynchronization pilot signal is greater than or equal to the presetthreshold, it indicates that quality of a channel on which thesynchronization pilot signal is sent and received meets a requirement ofa high frequency data transmission channel in the high frequency networkarchitecture. If a signal strength of a synchronization pilot signal isless than the preset threshold, it indicates that quality of a channelon which the synchronization pilot signal is sent and received cannotmeet a requirement of a high frequency data transmission channel in thehigh frequency network architecture, and the channel is an unavailablehigh frequency transmission channel.

For example, when the UE in FIG. 1 is in the coverage hole of theserving small cell, the UE needs to perform synchronous measurement onlyon the available beam pairs (the high frequency beam 2 of the UE, thehigh frequency beam 4 of the small cell 2) and (the high frequency beam3 of the UE, the high frequency beam 6 of the small cell 3), to selectan available target beam pair from the two beam pairs, that is, anoptimal beam pair can be determined as long as synchronous measurementis performed twice. Compared with an existing method in which an optimalbeam pair cannot be determined until synchronous measurement isperformed 120 times, this greatly reduces a quantity of times ofsynchronous measurement, and reduces a synchronous measurement time.Therefore, a time is saved for a small cell handover of the UE, a delayof re-accessing a high frequency network by the UE is reduced, and QoSof receiving a high frequency service by the UE is improved.

Further, as shown in FIG. 6A, the user equipment further includes:

a result reporting unit 606, configured to send a handover result to themacro base station, so that the macro base station instructs, accordingto the handover result, to perform data transmission between the targetsmall cell and the UE by using the available target beam pair, andinstructs the serving small cell to terminate data transmission with theUE, where the handover result includes the available target beam pairand a cell identifier of the target small cell, where

the receiving unit 603 is further configured to receive a datatransmission indication sent by the macro base station, to perform datatransmission with the target small cell by using the available targetbeam pair, where the data transmission indication is used to instruct toperform data transmission between the target small cell and the UE byusing the available target beam pair.

It can be known from the foregoing that, the user equipment 60 providedin this embodiment of the present application first performs synchronousmeasurement, to determine a set of small cells available for the UE;sends the set of available small cells to a macro base station; when theUE is in a coverage hole of a serving small cell, receives a firstsynchronization indication sent by the macro base station; performssynchronous measurement according to the first synchronizationindication, to select an available target beam pair; and hands over to atarget small cell corresponding to the available target beam pair. Inthis way, when the UE is in the coverage hole of the serving small cell,as long as the UE performs synchronous measurement on each availablebeam pair in the set of available small cells determined by the UE, theUE can select an optimal target small cell and target beam pair, andhands over to the target small cell, thereby reducing a delay ofre-accessing a high frequency network by the UE, improving QoS ofreceiving a high frequency service by the UE, and avoiding a problem inthe prior art that the UE needs to perform synchronous measurement onall beam pairs of all small cells, and a synchronous measurement time islong, causing a relatively large delay of re-accessing a high frequencynetwork by the UE, and relatively poor QoS of receiving a high frequencyservice by the UE.

In addition, an embodiment of the present application further provides amacro base station 70. As shown in FIG. 7, the macro base station 70 mayinclude:

a receiving unit 701, configured to: receive a set of available smallcells sent by UE, where the set of available small cells includes atleast one available small cell, and an available beam pair of eachavailable small cell, and the available beam pair consists of a highfrequency beam of the UE and a high frequency beam of the availablesmall cell; and

receive a coverage hole status indication sent by a serving small cellof the UE, where the coverage hole status indication is used to indicatethat the UE is in a coverage hole of the serving small cell; and

a sending unit 702, configured to: send a first synchronizationindication to the UE, and separately send a second synchronizationindication to each available small cell in the set of available smallcells, so that the UE performs synchronous measurement according to thefirst synchronization indication, to select an available target beampair, where the first synchronization indication is used to instruct theUE to perform synchronous measurement on the high frequency beam of theUE in each available beam pair, and the second synchronizationindication is used to instruct the available small cell to send asynchronization pilot signal to the UE on the high frequency beam in theavailable beam pair of the available small cell.

Further, the receiving unit 701 is further configured to receive ahandover result sent by the UE, where the handover result includes theavailable target beam pair and a cell identifier of the target smallcell; and

correspondingly, as shown in FIG. 7A, the macro base station furtherincludes:

an indication unit 703, configured to: instruct, according to thehandover result, to perform data transmission between the target smallcell and the UE by using the available target beam pair, and instructthe serving small cell to terminate data transmission with the UE.

Further, to ensure that the UE receives, as quickly as possible when theUE is in the coverage hole of the serving small cell, data transmittedby another small cell on a high frequency channel, and reduces aconnection delay of a high frequency data service, as shown in FIG. 7B,the macro base station further includes:

a grouping unit 704, configured to: before the indication unit 703instructs, according to the handover result, to perform datatransmission between the target small cell and the UE by using theavailable target beam pair, group, into one group, an available smallcell that is in the set of available small cells and that is in a samecoverage area of the macro base station, where

the sending unit 702 is further configured to: after the macro basestation receives a coverage hole status indication sent by the servingsmall cell of the UE, separately send, in a multicast form to each groupof available small cells, data that the UE requests to transmit.

In this way, before synchronous measurement is performed on a UE side todetermine the available target beam pair, the macro base station firstsends, in a multicast form to each small cell in the set of availablesmall cells, the data requested by the UE, so that after receiving thehandover result, the macro base station directly commands the targetsmall cell to perform data transmission to the UE, avoiding a waste oftime caused by that the data requested by the UE is transmitted to thetarget small cell by the macro base station only after the handoverresult is received, to enable the target small cell to perform datatransmission to the UE.

It can be known from the foregoing that, the macro base station 70provided in this embodiment of the present application receives a set ofavailable small cells sent by the UE, and after receiving a coveragehole status indication sent by a serving small cell, the macro basestation sends a first synchronization indication to the UE, andseparately sends a second synchronization indication to each availablesmall cell in the set of available small cells, so that the UE performssynchronous measurement according to the first synchronizationindication, to select an available target beam pair. The firstsynchronization indication is used to instruct the UE to performsynchronous measurement on a high frequency beam of the UE in eachavailable beam pair, and the second synchronization indication is usedto instruct the available small cell to send a synchronization pilotsignal to the UE on a high frequency beam in the available beam pair ofthe available small cell. In this way, after the macro base stationreceives information that is sent by the serving small cell and that theUE is in a coverage hole of the serving small cell, the macro basestation commands the UE to perform synchronous measurement on theavailable beam pair in the determined set of available small cells, sothat a quantity of times of synchronous measurement performed by the UEis greatly reduced, and a measurement time is reduced. Therefore, adelay of re-accessing a high frequency network by the UE is reduced, andhigh frequency QoS of the UE is improved.

In addition, an embodiment of the present application provides a smallcell 80. As shown in FIG. 8, the small cell 80 includes: a sending unit801, a receiving unit 802, and a data communications unit 803.

The sending unit 801 is configured to send a first synchronization pilotsignal to the UE, so that the UE measures the synchronization pilotsignal, and determines an available small cell of the UE and anavailable beam pair of the available small cell.

The available beam pair consists of a high frequency beam of theavailable small cell and a high frequency beam of the UE.

The receiving unit 802 is configured to receive a synchronizationindication sent by a macro base station, where the synchronizationindication instructs the available small cell to send a synchronizationpilot signal to the UE on the high frequency beam in the available beampair of the available small cell.

The sending unit 801 is further configured to send a secondsynchronization pilot signal to the UE according to the synchronizationindication, so that the UE performs synchronous measurement on the highfrequency beam of the UE in the available beam pair, determines anavailable target beam pair, and hands over from a current serving smallcell of the UE to a target small cell corresponding to the availabletarget beam pair.

The receiving unit 802 is further configured to receive a datatransmission indication sent by the macro base station.

The data communications unit 803 is configured to perform datatransmission with the UE by using the available target beam pair.

Further, the receiving unit 802 is further configured to: before thereceiving unit receives the data transmission indication sent by themacro base station, receive data that is sent by the macro base stationin a multicast form and that the UE requests to transmit.

Further, when the small cell 80 is the current serving small cell of theUE, as shown in FIG. 8A, the small cell 80 further includes:

a determining unit 804, configured to determine whether the UE is in acoverage hole of the current serving small cell, where

the sending unit 801 is further configured to send a coverage holestatus indication to the macro base station, where the coverage holestatus indication is used to indicate whether the UE is in the coveragehole of the small cell; and

the receiving unit 802 is further configured to: when the UE is in thecoverage hole of the current serving small cell, receive, by the smallcell, indication information sent by the macro base station, toterminate data transmission with the UE.

Further, the determining unit 804 is specifically configured to:

if it is continuously determined at least twice that an uplink of the UEis incorrectly demodulated, determine that the UE is in the coveragehole of the current serving small cell; or

if a received reference signal received power RSRP or reference signalreceived quality RSRQ sent by the UE is less than a preset threshold,determine that the UE is in the coverage hole of the current servingsmall cell, where the preset threshold is set as required, and is notlimited in this embodiment of the present application; or

if a response message of the UE is not received continuously at leasttwice after a request message is sent to the UE, determine that the UEis in the coverage hole of the small cell.

It can be known from the foregoing that, the small cell 80 provided inthis embodiment of the present application sends a first synchronizationpilot signal to the UE, so that the UE measures the synchronizationpilot signal, and determines an available small cell of the UE and anavailable beam pair of the available small cell, where the availablebeam pair consists of a high frequency beam of the available small celland a high frequency beam of the UE; the available small cell receives asynchronization indication sent by a macro base station, where thesynchronization indication instructs the available small cell to send asynchronization pilot signal to the UE on the high frequency beam in theavailable beam pair of the available small cell; the available smallcell sends a second synchronization pilot signal to the UE according tothe synchronization indication, so that the UE performs synchronousmeasurement on the high frequency beam of the UE in the available beampair, determines an available target beam pair, and hands over from acurrent serving small cell of the UE to a target small cellcorresponding to the available target beam pair; the target small cellreceives a data transmission indication sent by the macro base station,to perform data transmission with the UE by using the available targetbeam pair. In this way, when the UE is in a coverage hole of the servingsmall cell, the UE needs to perform synchronous measurement only on theavailable beam pair in a set of available small cells, greatly reducinga quantity of synchronization times. Therefore, a delay of accessing ahigh frequency network by the UE is reduced, and QoS of receiving a highfrequency service by the UE is improved.

In addition, an embodiment of the present application provides aninter-small cell handover system 90. As shown in FIG. 9, the inter-smallcell handover system 90 may include user equipment 60, a macro basestation 70, at least one serving small cell 80, and at least oneavailable small cell 0, where

the user equipment 60, the macro base station 70, and the small cell 80are respectively the same as the user equipment 60, the macro basestation 70, and the small cell 80 that are in the foregoing, and are notdescribed herein again.

It can be known from the foregoing that, by means of the inter-smallcell handover system 90 provided in this embodiment of the presentapplication, the UE 60 performs synchronous measurement, determines aset of small cells available for the UE 60, and sends the set ofavailable small cells to the macro base station 70; when determiningthat the UE is in a coverage hole of a serving small cell, the smallcell 80 sends a coverage hole status indication to the macro basestation 70; the macro base station 70 sends a first synchronizationindication to the UE, and sends a second synchronization indication toan available small cell in the set of available small cells; the UE 60performs synchronous measurement, determines an available target beampair, and hands over to a target small cell corresponding to theavailable target beam pair. In this way, when the UE is in the coveragehole of the serving small cell, as long as the UE performs synchronousmeasurement on each available beam pair in the set of available smallcells determined by the UE, the UE can select an optimal target smallcell and target beam pair, and hands over to the target small cell,thereby reducing a delay of re-accessing a high frequency network by theUE, improving QoS of receiving a high frequency service by the UE, andavoiding a problem in the prior art that the UE needs to performsynchronous measurement on all beam pairs of all small cells, and asynchronous measurement time is long, causing a relatively large delayof re-accessing a high frequency network by the UE, and relatively poorQoS of receiving a high frequency service by the UE.

In addition, an embodiment of the present application further providesuser equipment 100. As shown in FIG. 10, the user equipment 100 mayinclude: a communications unit 1001, a processor 1002, a memory 1003,and at least one communications bus 1004, which is configured toimplement a connection and mutual communication between theseapparatuses.

The processor 1002 may be a central processing unit (CPU).

The memory 1003 may be a volatile memory, such as a random-access memoryRAM for short); or a non-volatile memory, such as a read-only memory(ROM), a flash memory, a hard disk drive (HDD) or a solid-state drive(SSD); or a combination of the foregoing types of memories; and providesinstructions and data for the processor 1002.

The processor 1002 is configured to perform synchronous measurement, todetermine a set of small cells available for the UE.

The set of available small cells includes at least one available smallcell, and an available beam pair of each available small cell, and theavailable beam pair consists of a high frequency beam of the UE and ahigh frequency beam of the available small cell.

The communications unit 1001 is configured to: send the set of availablesmall cells to a macro base station; and

when the UE is in a coverage hole of a current serving small cell,receive a first synchronization indication sent by the macro basestation.

The first synchronization indication is used to instruct the UE toperform synchronous measurement on the high frequency beam of the UE ineach available beam pair.

The processor 1002 is further configured to perform synchronousmeasurement according to the first synchronization indication, to selectan available target beam pair; and

hand over to a target small cell corresponding to the available targetbeam pair.

Further, the processor 1002 is specifically configured to:

send a synchronous measurement request to the macro base station, sothat the macro base station instructs all small cells in a coverage areaof the macro base station to send synchronization pilot signals to theUE;

separately receive, on each high frequency beam of the UE, thesynchronization pilot signal sent by all the small cells on each highfrequency beam of the small cells;

separately measure a signal strength of each synchronization pilotsignal, and sort the signal strengths in descending order; and

determine a small cell sending a synchronization pilot signal having ahighest signal strength as a serving small cell of the UE; and determineat least one small cell sending a synchronization pilot signal having asecond highest signal strength as an available small cell in the set ofsmall cells available for the UE.

The signal strength may be a measured RSRP value of a reference signalof a current neighboring small cell or a measured RSRQ value of areference signal of a current neighboring small cell.

For example, as shown in FIG. 1, after performing synchronousmeasurement, the UE determines the small cell 1 as the serving smallcell of the UE, determines that available small cells in the set ofavailable small cells are the small cell 2 and the small cell 3,determines that an available beam pair corresponding to the small cell 2is (a high frequency beam 2 of the UE, a high frequency beam 4 of thesmall cell 2), and determines that an available beam pair correspondingto the small cell 3 is (a high frequency beam 3 of the UE, a highfrequency beam 6 of the small cell 3).

Further, the processor 1002 is specifically configured to:

measure a signal strength of a synchronization pilot signal on the highfrequency beam of the UE in each available beam pair; and

if the signal strength corresponding to the synchronization pilot signalhaving the highest signal strength is greater than or equal to a presetthreshold, select, as the available target beam pair, a beam pairconsisting of a high frequency beam, of the UE, on which thesynchronization pilot signal having the highest signal strength isreceived and a high frequency beam of an available small cell sendingthe synchronization pilot signal having the highest signal strength.

The preset threshold is set as required, and is not limited in thisembodiment of the present application. If a signal strength of asynchronization pilot signal is greater than or equal to the presetthreshold, it indicates that quality of a channel on which thesynchronization pilot signal is sent and received meets a requirement ofa high frequency data transmission channel in the high frequency networkarchitecture. If a signal strength of a synchronization pilot signal isless than the preset threshold, it indicates that quality of a channelon which the synchronization pilot signal is sent and received cannotmeet a requirement of a high frequency data transmission channel in thehigh frequency network architecture, and the channel is an unavailablehigh frequency transmission channel.

For example, when the UE in FIG. 1 is in the coverage hole of theserving small cell, the UE needs to perform synchronous measurement onlyon the available beam pairs (the high frequency beam 2 of the UE, thehigh frequency beam 4 of the small cell 2) and (the high frequency beam3 of the UE, the high frequency beam 6 of the small cell 3), to selectan available target beam pair from the two beam pairs, that is, anoptimal beam pair can be determined as long as synchronous measurementis performed twice. Compared with an existing method in which an optimalbeam pair cannot be determined until synchronous measurement isperformed 120 times, this greatly reduces a quantity of times ofsynchronous measurement, and reduces a synchronous measurement time.Therefore, a time is saved for a small cell handover of the UE, a delayof re-accessing a high frequency network by the UE is reduced, and QoSof receiving a high frequency service by the UE is improved.

Further, the communications unit 1001 is further configured to: send ahandover result to the macro base station, so that the macro basestation instructs, according to the handover result, to perform datatransmission between the target small cell and the UE by using theavailable target beam pair, and instructs the serving small cell toterminate data transmission with the UE, where the handover resultincludes the available target beam pair and a cell identifier of thetarget small cell; and

receive a data transmission indication sent by the macro base station,to perform data transmission with the target small cell by using theavailable target beam pair, where the data transmission indication isused to instruct to perform data transmission between the target smallcell and the UE by using the available target beam pair.

It can be known from the foregoing that, the user equipment 100 providedin this embodiment of the present application first performs synchronousmeasurement, to determine a set of small cells available for the UE;sends the set of available small cells to a macro base station; when theUE is in a coverage hole of a serving small cell, receives a firstsynchronization indication sent by the macro base station; performssynchronous measurement according to the first synchronizationindication, to select an available target beam pair; and hands over to atarget small cell corresponding to the available target beam pair. Inthis way, when the UE is in the coverage hole of the serving small cell,as long as the UE performs synchronous measurement on each availablebeam pair in the set of available small cells determined by the UE, theUE can select an optimal small cell and beam pair, access the selectedsmall cell, and continuously perform high frequency data transmission byusing the selected beam pair, thereby reducing a synchronous measurementtime, further reducing a delay of re-accessing a high frequency networkby the UE, improving QoS of receiving a high frequency service by theUE, and avoiding a problem in the prior art that the UE needs to performsynchronous measurement on all beam pairs of all small cells, and asynchronous measurement time is long, causing a relatively large delayof re-accessing a high frequency network by the UE, and relatively poorQoS of receiving a high frequency service by the UE.

In addition, an embodiment of the present application further provides amacro base station 110. As shown in FIG. 11, the macro base station 110may include: a communications unit 1101, a processor 1102, a memory1103, and at least one communications bus 1104, which is configured toimplement a connection and mutual communication between theseapparatuses.

The processor 1102 may be a central processing unit (CPU).

The memory 1103 may be a volatile memory, such as a random-access memory(RAM); or a non-volatile memory, such as a read-only memory (ROM), aflash memory, a hard disk drive (HDD) or a solid-state drive (SSD); or acombination of the foregoing types of memories; and providesinstructions and data for the processor 1102.

The communications unit 1101 is configured to: receive a set ofavailable small cells sent by the UE, where the set of available smallcells includes at least one available small cell, and an available beampair of each available small cell, and the available beam pair consistsof a high frequency beam of the UE and a high frequency beam of theavailable small cell;

receive a coverage hole status indication sent by a serving small cellof the UE, where the coverage hole status indication is used to indicatethat the UE is in a coverage hole of the serving small cell; and

send a first synchronization indication to the UE, and separately send asecond synchronization indication to each available small cell in theset of available small cells, so that the UE performs synchronousmeasurement according to the first synchronization indication, to selectan available target beam pair, where the first synchronizationindication is used to instruct the UE to perform synchronous measurementon the high frequency beam of the UE in each available beam pair, andthe second synchronization indication is used to instruct the availablesmall cell to send a synchronization pilot signal to the UE on the highfrequency beam in the available beam pair of the available small cell.

Further, the communications unit 1101 is further configured to receive ahandover result sent by the UE, where the handover result includes theavailable target beam pair and a cell identifier of the target smallcell; and

correspondingly, the processor 1102 is configured to: instruct,according to the handover result, to perform data transmission betweenthe target small cell and the UE by using the available target beampair, and instruct the serving small cell to terminate data transmissionwith the UE.

Further, to ensure that the UE receives, as quickly as possible when theUE is in the coverage hole of the serving small cell, data transmittedby another small cell on a high frequency channel, and reduces aconnection delay of a high frequency data service, the processor 1102 isfurther configured to: before the indication unit instructs, accordingto the handover result, to perform data transmission between the targetsmall cell and the UE by using the available target beam pair, group,into one group, an available small cell that is in the set of availablesmall cells and that is in a same coverage area of the macro basestation; and

the communications unit 1101 is further configured to: after the macrobase station receives a coverage hole status indication sent by theserving small cell of the UE, separately send, in a multicast form toeach group of available small cells, data that the UE requests totransmit.

In this way, before synchronous measurement is performed on a UE side todetermine the available target beam pair, the macro base station firstsends, in a multicast form to each small cell in the set of availablesmall cells, the data requested by the UE, so that after receiving thehandover result, the macro base station directly commands the targetsmall cell to perform data transmission to the UE, avoiding a waste oftime caused by that the data requested by the UE is transmitted to thetarget small cell by the macro base station only after the handoverresult is received, to enable the target small cell to perform datatransmission to the UE.

It can be known from the foregoing that, the macro base station 110provided in this embodiment of the present application receives a set ofavailable small cells sent by the UE, and after receiving a coveragehole status indication sent by a serving small cell, the macro basestation sends a first synchronization indication to the UE, andseparately sends a second synchronization indication to each availablesmall cell in the set of available small cells, so that the UE performssynchronous measurement according to the first synchronizationindication, to select an available target beam pair. The firstsynchronization indication is used to instruct the UE to performsynchronous measurement on a high frequency beam of the UE in eachavailable beam pair, and the second synchronization indication is usedto instruct the available small cell to send a synchronization pilotsignal to the UE on a high frequency beam in the available beam pair ofthe available small cell. In this way, after the macro base stationreceives information that is sent by the serving small cell and that theUE is in a coverage hole of the serving small cell, the macro basestation commands the UE to perform synchronous measurement on theavailable beam pair in the determined set of available small cells, sothat a quantity of times of synchronous measurement performed by the UEis greatly reduced, and a measurement time is reduced. Therefore, adelay of re-accessing a high frequency network by the UE is reduced, andhigh frequency QoS of the UE is improved.

In addition, an embodiment of the present application further provides asmall cell 120. As shown in FIG. 12, the small cell 120 may include: acommunications unit 1201, a processor 1202, a memory 1203, and at leastone communications bus 1204, which is configured to implement aconnection and mutual communication between these apparatuses.

The processor 1202 may be a central processing unit (CPU).

The memory 1203 may be a volatile memory, such as a random-access memory(RAM); or a non-volatile memory, such as a read-only memory (ROM), aflash memory, a hard disk drive (HDD) or a solid-state drive (SSD); or acombination of the foregoing types of memories; and providesinstructions and data for the processor 1202.

The processor 1202 is configured to determine that UE is in a coveragehole of a serving small cell.

The communications unit 1201 sends a first synchronization pilot signalto the UE, so that the UE measures the synchronization pilot signal, anddetermines an available small cell of the UE and an available beam pairof the available small cell.

The available beam pair consists of a high frequency beam of theavailable small cell and a high frequency beam of the UE.

The communications unit 1201 is further configured to: receive asynchronization indication sent by a macro base station, where thesynchronization indication instructs the available small cell to send asynchronization pilot signal to the UE on the high frequency beam in theavailable beam pair of the available small cell;

send a second synchronization pilot signal to the UE according to thesynchronization indication, so that the UE performs synchronousmeasurement on the high frequency beam of the UE in the available beampair, determines an available target beam pair, and hands over from acurrent serving small cell of the UE to a target small cellcorresponding to the available target beam pair;

receive a data transmission indication sent by the macro base station;and

perform data transmission with the UE by using the available target beampair.

Further, the communications unit 1201 is further configured to: beforethe receiving unit receives the data transmission indication sent by themacro base station, receive data that is sent by the macro base stationin a multicast form and that the UE requests to transmit.

Further, when the small cell 120 is the current serving small cell ofthe UE, the processor 1202 is configured to determine whether the UE isin a coverage hole of the current serving small cell; and

the communications unit 1201 is further configured to: send a coveragehole status indication to the macro base station, where the coveragehole status indication is used to indicate whether the UE is in thecoverage hole of the small cell; and

when the UE is in the coverage hole of the current serving small cell,receive, by the small cell, indication information sent by the macrobase station, to terminate data transmission with the UE.

Further, the processor 1202 is specifically configured to:

if it is continuously determined at least twice that an uplink of the UEis incorrectly demodulated, determine that the UE is in the coveragehole of the current serving small cell; or

if a received reference signal received power RSRP or reference signalreceived quality RSRQ sent by the UE is less than a preset threshold,determine that the UE is in the coverage hole of the current servingsmall cell, where the preset threshold is set as required, and is notlimited in this embodiment of the present application; or

if a response message of the UE is not received continuously at leasttwice after a request message is sent to the UE, determine that the UEis in the coverage hole of the small cell.

It can be known from the foregoing that, the small cell 120 provided inthis embodiment of the present application sends a first synchronizationpilot signal to UE, so that the UE measures the synchronization pilotsignal, and determines an available small cell of the UE and anavailable beam pair of the available small cell, where the availablebeam pair consists of a high frequency beam of the available small celland a high frequency beam of the UE; the available small cell receives asynchronization indication sent by a macro base station, where thesynchronization indication instructs the available small cell to send asynchronization pilot signal to the UE on the high frequency beam in theavailable beam pair of the available small cell; the available smallcell sends a second synchronization pilot signal to the UE according tothe synchronization indication, so that the UE performs synchronousmeasurement on the high frequency beam of the UE in the available beampair, determines an available target beam pair, and hands over from acurrent serving small cell of the UE to a target small cellcorresponding to the available target beam pair; the target small cellreceives a data transmission indication sent by the macro base station,to perform data transmission with the UE by using the available targetbeam pair. In this way, when the UE is in a coverage hole of the servingsmall cell, the UE needs to perform synchronous measurement only on theavailable beam pair in a set of available small cells, greatly reducinga quantity of synchronization times. Therefore, a delay of accessing ahigh frequency network by the UE is reduced, and QoS of receiving a highfrequency service by the UE is improved.

In addition, an embodiment of the present application provides aninter-small cell handover system 130. As shown in FIG. 13, theinter-small cell handover system 130 may include user equipment 100, amacro base station 110, and at least one small cell 120, where

the user equipment 100, the macro base station 110, and the small cell120 are respectively the same as the user equipment 100, the macro basestation 110, and the small cell 120 that are in the foregoing, and arenot described herein again.

It can be known from the foregoing that, by means of the inter-smallcell handover system 130 provided in this embodiment of the presentapplication, the UE 100 performs synchronous measurement, determines aset of small cells available for the UE 100, and sends the set ofavailable small cells to the macro base station 110; when determiningthat the UE is in a coverage hole of a serving small cell, the smallcell 120 sends a coverage hole status indication to the macro basestation 110; the macro base station 110 sends a first synchronizationindication to the UE, and separately sends a second synchronizationindication to an available small cell in the set of available smallcells; the UE 100 performs synchronous measurement, determines anavailable target beam pair, and hands over to a target small cellcorresponding to the available target beam pair. In this way, when theUE is in the coverage hole of the serving small cell, as long as the UEperforms synchronous measurement on each available beam pair in the setof available small cells determined by the UE, the UE can select anoptimal small cell and beam pair, access the selected small cell, andcontinuously perform high frequency data transmission by using theselected beam pair, thereby reducing a synchronous measurement time,further reducing a delay of re-accessing a high frequency network by theUE, improving QoS of receiving a high frequency service by the UE, andavoiding a problem in the prior art that the UE needs to performsynchronous measurement on all beam pairs of all small cells, and asynchronous measurement time is long, causing a relatively large delayof re-accessing a high frequency network by the UE, and relatively poorQoS of receiving a high frequency service by the UE.

In the several embodiments provided in the present application, itshould be understood that the disclosed system, mobile terminal, andmethod may be implemented in other manners. For example, the describedmobile terminal embodiment is merely an example. For example, the unitdivision is merely logical function division and may be other divisionin actual implementation. For example, a plurality of units orcomponents may be combined or integrated into another system, or somefeatures may be ignored or not performed. In addition, the displayed ordiscussed mutual couplings or direct couplings or communicationconnections may be implemented through some interfaces. The indirectcouplings or communication connections between the mobile terminals orunits may be implemented in electronic, mechanical, or other forms.

The units described as separate parts may or may not be physicallyseparate, and parts displayed as units may or may not be physical units,may be located in one position, or may be distributed on a plurality ofnetwork units. Some or all of the units may be selected according toactual needs to achieve the objectives of the solutions of theembodiments. In addition, functional units in the embodiments of thepresent application may be integrated into one processing unit, or eachof the units may exist alone physically, or two or more units areintegrated into one unit. The integrated unit may be implemented in aform of hardware, or may be implemented in a form of hardware inaddition to a software functional unit.

When the foregoing integrated unit is implemented in a form of asoftware functional unit, the integrated unit may be stored in acomputer-readable storage medium. The software functional unit is storedin a storage medium and includes several instructions for instructing acomputer device (which may be a personal computer, a server, or anetwork device) to perform some of the steps of the methods described inthe embodiments of the present application. The foregoing storage mediumincludes: any medium that can store program code, such as a USB flashdrive, a removable hard disk, a read-only memory (ROM), a random accessmemory (RAM), a magnetic disk, or an optical disc.

Finally, it should be noted that, the foregoing embodiments are merelyintended for describing the technical solutions of the presentapplication, but not for limiting the present application. Although thepresent application is described in detail with reference to theforegoing embodiments, persons of ordinary skill in the art shouldunderstand that they may still make modifications to the technicalsolutions described in the foregoing embodiments or make equivalentreplacements to some technical features thereof, without departing fromthe spirit and scope of the technical solutions of the embodiments ofthe present application.

What is claimed is:
 1. A method for determining an available small cellfor a user equipment (UE), the method comprising: sending, by the UE, asynchronous measurement request to a macro base station, which causesthe macro base station to instruct each small cell in a coverage area ofthe macro base station to send synchronization pilot signals to the UE;receiving, by the UE on a high frequency beam of the UE, thesynchronization pilot signal sent by each of the small cells on a highfrequency beam of each of the small cells; measuring, by the UE, asignal strength of each synchronization pilot signal, and sorting thesignal strengths in descending order; determining, by the UE, a firstsmall cell sending a first synchronization pilot signal having a highestsignal strength as a serving small cell of the UE; determining, by theUE, at least one second small cell sending a second synchronizationpilot signal having a second highest signal strength as an availablesmall cell in a set of available small cells for the UE; sending, by theUE, information corresponding to the set of available small cells to themacro base station; and in response to determining that the UE is in acoverage hole of a current serving small cell, receiving, by the UE, afirst synchronization indication from the macro base station, whereinthe first synchronization indication is used to instruct the UE toperform synchronous measurement on the high frequency beam of the UEwith the high frequency beam of each small cell in the set of availablesmall cells.
 2. The method according to claim 1, further comprising:performing, by the UE, synchronous measurement according to the firstsynchronization indication; selecting, by the UE, an available smallcell in the set of available small cells based on the synchronousmeasurement.
 3. The method according to claim 2, further comprises:measuring, by the UE, a signal strength of a synchronization pilotsignal on the high frequency beam of the UE with each small cell in theset of available small cells; and if the signal strength correspondingto the synchronization pilot signal having the highest signal strengthis greater than or equal to a preset threshold, selecting, as anavailable target beam pair, a beam pair consisting of the high frequencybeam of the UE, on which the synchronization pilot signal having thehighest signal strength is received and the high frequency beam of theavailable small cell sending the synchronization pilot signal having thehighest signal strength.
 4. A user equipment (UE), comprising: one ormore memories configured to store instructions; and one or moreprocessors coupled to the one or more memories and configured to executethe instructions to cause the UE to: send a synchronous measurementrequest to a macro base station, which causes the macro base station toinstruct each small cell in a coverage area of the macro base station tosend synchronization pilot signals to the UE; receive, on a highfrequency beam of the UE, the synchronization pilot signal sent by eachof the small cells on a high frequency beam of each of the small cells;measure a signal strength of each synchronization pilot signal, and sortthe signal strengths in descending order; determine a first small cellsending a first synchronization pilot signal having a highest signalstrength as a serving small cell of the UE; determine at least onesecond small cell sending a synchronization pilot signal having a secondhighest signal strength as an available small cell in a set of availablesmall cells for the UE; send information corresponding to the set ofavailable small cells to a macro base station; and in response todetermining that the UE is in a coverage hole of a current serving smallcell, receive a first synchronization indication from the macro basestation, wherein the first synchronization indication is used toinstruct the UE to perform synchronous measurement on the high frequencybeam of the UE with the high frequency beam of each small cell in theset of available small cells.
 5. The user equipment according to claim4, wherein the one or more processors are further configured to executethe program instructions to cause the UE to: perform synchronousmeasurement according to the first synchronization indication; andselect an available small cell in the set of available small cells basedon the synchronous measurement.
 6. The user equipment according to claim5, wherein the one or more processors are further configured to executethe program instructions to cause the UE to: measure a signal strengthof a synchronization pilot signal on the high frequency beam of the UEwith each small cell in the set of available small cells; and if thesignal strength corresponding to the synchronization pilot signal havingthe highest signal strength is greater than or equal to a presetthreshold, select, as an available target beam pair, a beam pairconsisting of the high frequency beam of the UE, on which thesynchronization pilot signal having the highest signal strength isreceived and the high frequency beam of the available small cell sendingthe synchronization pilot signal having the highest signal strength.